Photolithographically patterned micro spring structures (sometimes referred to as “microsprings”) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical spring structure includes a spring finger having an anchor portion secured to a substrate, and a free (cantilevered) portion extending from the anchored portion over the substrate. The spring finger is formed from a stress-engineered film (i.e., a (e.g., metal) film fabricated such that portions closer to the underlying substrate have a higher internal compressive stress than its portions located farther from the substrate) that is at least partially formed on a release material layer. The free portion of the spring finger bends away from the substrate when the release material located under the free portion is etched away. The internal stress gradient is produced in the spring by layering, e.g., different metals having the desired stress characteristics, or by using a single metal by altering the fabrication parameters. Such spring structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. For example, when utilized in a probe card application, the tip of the free portion is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
A problem associated with the manufacture of various products including micro spring structures is that the conventional micro spring fabrication processes require either physically locating the product into the associated micro spring structure manufacturing tool (e.g., a sputter deposition chamber), or pre-fabricating the micro spring structures on a substrate, and then securing the substrate to the product (i.e., such that the substrate is located between the spring structures and the product). For example, in order to produce a flexible cable having micro spring structures located on the cable's conductors, the flexible cable must either be inserted into the manufacturing tool, or the micro spring structures must be pre-fabricated on a conductive substrate that is then mounted onto the exposed conductors of the cable. Placing the flexible cable in the manufacturing tool increases production complexity (i.e., the flexible cable must be able to survive the fabrication process), and significantly decreases production efficiency due to the large amount of space needed to accommodate the flexible cable. Conversely, pre-fabrication requires releasing the micro spring structures, and then transferring the released spring structures to an assembly point, this process greatly increasing the risk of damaging the relatively fragile micro-spring structures. Further, because the substrate on which the spring structures are formed is mounted on the selected product, either the substrate must be formed with integrated conductive and/or insulated regions, or the substrate must be diced into very small pieces prior to the mounting process. In either case, the cost and complexity of producing products having micro spring structures is greatly increased, thereby significantly reducing manufacturing efficiencies and greatly increasing manufacturing costs.
What is needed is a method for transferring pre-fabricated micro spring structures that both protects the relatively fragile spring structure, and facilitates a relatively simple and reliable process for mounting the spring structures onto a selected product.